1. Field of the Invention
Electrodialysis has become an accepted process and apparatus for transferring electrolytes from one electrolytically conducting solution to another. The state of the art is well-described in pages 726 through 738, Volume 8, Kirk-Othmer Encyclopedia of Chemical Technology, Third Edition, John Wiley and Sons, New York, 1979. Typically one hundred or more pairs of spaced electrolytically conducting barriers are arrayed between a pair of electrodes. In each pair of barriers, one barrier has a transport number for ions of one sign substantially greater than that of the above mentioned solutions and of the other barrier in said pair. Said substantially selective barriers alternate in the array with said less selective barriers. Solutions of electrolyte(s) are introduced into the spaces between the barriers and between the electrodes and their adjacent barriers. A substantially direct current electric potential is applied between the electrodes causing electrolyte to be transferred from every other space to the intervening spaces. Electrolyte enriched solution is withdrawn from the latter spaces and electorlyte depleted solution from the former.
In practice charged colloids or high molecular weight molecules (such as proteins) are driven by the direct current electric potential toward at least one of the barriers at which they may form highly viscous films which interfere with convective and diffusive electrolyte transfer to the barrier. Such colloids and high molecular weight molecules may also form more or less insoluble coagulum or precipitates on or within the surface of the barrier, which also interfere with electrolyte transfer and/or affect the physical integrity of the barrier.
Such phenomena are encountered for example during the electrodialysis of cheese whey concentrates, a popular application of electrodialysis; during the electrodialysis of other protein solutions; during the electrodialytic desalting of primary, secondary and even tertiary treated sewage effluent; and during the electrodialytic desalting of surface waters. Such undesirable effects may be alleviated by a combination of: high fluid shear at the affected surface of the barrier; symmetric, periodic reversal of the direction of the direct current potential; and/or periodic cleaning of the affected barrier in situ.
When barriers having a substantial anion transport number are used for electrodialytic desalting which barriers have a significant number of weakly basic amine groups near the interface with the space which is being depleted or when said interface has sorbed or deposited materials which interfere with convective and/or diffusive transport of electrolyte to the barrier interface and/or which catalyze the dissociation of water into hydronium cations and hydroxide anions, then at appreciable current densities a significant fraction of electric current passing through the barrier may be carried by hydroxide anions.
As a result the pH may become quite high at the interface between such barrier and the space being electrolytically enriched.
Electrolytes (e.g. calcium bicarbonate) which are not appreciably soluable at high pH's may then precipitate at or in such interface. Even in the absence of pH changes the solubility of some electrolytes (e.g. calcium sulfate) may be exceeded in the enriching space, resulting in deposits on the barrier.
These solubility and pH effects may be alleviated by a combination of: chemical additions to the enriching space (e.g. sodium hexa metaphosphate or acid resp.); high fluid shear at the affected surface of the barrier; symmetric, periodic reversal of the direction of the electric current potential and/or periodic cleaning of the affected surface in situ.
It has now been found that the above mentioned depositions on the electrolytically conducting barriers of electrodialysis can be substantially decreased by including in the depleting space a plurality of fluid permeable tubules. It is an object of the present invention to substantially reduce depositions on the barriers of electrodialysis apparatus by including in the depleting spaces of such apparatus a plurality of fluid permeable tubules and feeding to the lumen of such tubules an appropriate fluid, for example, at least part of the feed intended for the depleting space with at least part of such fluid passing through the walls of the tubules into the depleting space(s).
These and other objects will become apparent from the following description of the invention.
2. Description of the Prior Art:
U.S. Pat. No. 3,784,460 issued Jan. 8, 1974 describes an apparatus for treating an electrodeposition bath which comprises in combination an electrodeposition bath tank, an electrodialysis compartment dividing the electrodeposition bath tank into a coating zone and a counterelectrode compartment by a single electrolytically conducting barrier and a reverse osmosis or ultrafiltration unit external to the electrodeposition bath tank. The electrodialysis compartment supplies substantially direct current from a counterelectrode to the electrodepositing working electrode while simultaneously removing part of the counterions from the electrodepositing bath. The reverse osmosis or ultrafiltration unit external to the electrodeposition bath tank removes respectively, water and low molecule weight solutes from the electrodeposition bath. The reverse osmosis or ultrafiltration unit is not within the electric field of the electrodialysis/electrodeposition apparatus or between any pair of membranes in such apparatus. There is no synergistic relationship between the electrodialysis compartment and the reverse osmosis or ultrafiltration unit. The overall apparatus is a mere agglomeration of an electrodialysis/electrodeposition unit with a reverse osmosis or ultrafiltration unit which could be entirely decoupled and separated from each other by a great distance. As a consequence, the verse osmosis/ultrafiltration unit does not affect the performance of the electrodialysis cell.
U.S. Pat. No. 3,905,886 issued Sept. 16, 1975 describes a combination electrodialysis and ultrafiltration cell pack having planar ultrafiltration membranes interposed between a pair of planar ion selective membranes. A solution (such as cheese whey) to be concentrated and demineralized is fed under pressure to one side of the ultrafiltration membrane and a substantially direct current electric field is applied across the cell pack. Ions of one polarity pass through a first ion-selective membrane (e.g. a cation selective membrane) adjacent the feed cell while oppositely charged ions (resp. anions) penetrate the ultrafiltration membrane together with the ultrafiltration permeate. Finally the latter ions penetrate the other ion-selective membranes (resp. anion selective). Three process streams exit the system: in the case of cheese whey a protein concentrated, partially demineralized feed solution; a partially demineralized substantially protein free, lactose bearing permeate and a substantially protein-free, lactose-free stream containing the electrolytes removed from the other two streams. The apparatus is not practical however since a substantial pressure (up to several bars) must be applied across the ultrafiltration membrane to achieve ultrafiltration flux rates commensurate with economic electrodialysis production rates (20 to 200 gals/ft day). The substantially hydraulically impermeable ion selective membrane which forms the second wall of the feed cell is subjected to the same pressure and is therefore forced toward the other ion-selective membrane which sees only low pressure, requiring special structure in the space between the ion-selective membranes. It is common practice in the design of electrodialysis apparatus to have substantially no pressure difference from one side of an ion selective membrane to the other in order to minimize crossleak (e.g. of protein and lactose) from an electrolyte depleting space to an electrolyte enriching space. The high pressure differentials required for the ultrafiltration membrane in the apparatus of U.S. Pat. No. 3,905,886 make the control of cross-leaks extremely difficult. Further the high pressures make it necessary to use a very much more massive mechanism to close the cell pack and prevent leaks to the outside. For example in a typical apparatus having an active area of about 3000 cm2 and using a pressure differential of 3 bars to drive the ultrafiltration membrane the extra force on the closure mechanism is about 9 tonnes.
U.S. Pat. Nos. 4,043,896 issued Aug. 23, 1977 and 4,123,342 issued Oct. 31, 1978 are similar to U.S. Pat. No. 3,905,886 described above except that either the anion-selective or the cation-selective membrane is replaced with a substantially non-selective membrane. The apparatus described suffers from the same practical problems as that of U.S. Pat. No. 3,905,886.
All of the prior art apparatuses summarized above either suffer from high costs associated with the problem of handling the high pressures applied to both the ultrafiltration and electrodialysis membranes or do not substantially protect the electrodialysis membranes from deposits.
It is therefore a further objective of this invention to provide apparatus and processes having lower cost, which apparatus and processes do substantially protect the electrodialysis barriers from deposits and in which the membrane filtration/ultrafiltration/reverse osmosis driving forces are not applied to the electrodialysis barriers.